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Long focal lengths from small diameter lenses and mirrors are best for any sort of accurate viewing due to the limitations of the curvature on the lenses or mirrors because of chromatic and spherical aberation. This gets more pronounced as the F ratio reduces.
The common SCT's are in fact Low F ratio telescopes being in the order of F2 or F3 but have in fact have had the focal length increased with a convex secondary mirror (Barlow) to make them generally F10. Special corrections in the light path are made by corrector plates and figureing to make the performance like an F10 telecope.
Barry
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